This application relates to Bragg gratings in optical waveguides, and more specifically, to fabrication of such Bragg gratings.
A Bragg grating may be formed in a waveguide such as an optical fiber or a dielectric waveguide by producing a periodic spatial pattern along the waveguide. In a fiber Bragg grating, for example, the product of the effective refractive index, n, of the fiber and the spatial period, xcex9, of the spatial pattern, i.e., the grating parameter, may be either a constant everywhere along the fiber or a monotonic function of the position along the fiber. A spectral component at a wavelength xcex in an input optical wave to the fiber grating, when satisfying the Bragg resonance condition, xcex=2nxcex9, at one location of the fiber Bragg grating, interacts with the grating and is reflected back. Spectral components that do not satisfy the Bragg resonance condition at any location in the fiber grating transmit through the fiber grating.
The periodic spatial pattern may be a spatial modulation of either or both of the amplitude and the phase of the refractive index of the fiber core. In one implementation, the fiber core may be made photosensitive by implanting a photosensitive material in the fiber core. Hence, exposing the fiber core to a desired radiation pattern may be performed to imprint the desired spatial pattern in the fiber core. The desired radiation pattern may be formed by interference of two coherent radiation beams. The two beams may be generated by, e.g., using either a fixed phase mask or an holographic interferometer.
The present disclosure includes optical interferometric techniques and systems for fabricating waveguide Bragg gratings in photosensitive waveguides such as optical fibers by using an acousto-optic element to generate and control the radiation pattern. A high resolution in the spatial features of the spatial pattern may be achieved by operating and controlling the acousto-optic element. The radiation pattern may be programmable in that different radiation patterns may be produced by controlling the acousto-optic element. A scanning mechanism is provided to spatially scan the fiber relative to the interference pattern so that different sections of the fiber are exposed to form grating patterns which may change with position along the fiber.
One embodiment of the writing system uses two intercepting acoustic waves to produce an acoustic interference pattern to diffract a CW or pulsed input optical beam into two separate but mutually coherent diffraction beams to produce the desired radiation pattern. In another embodiment, a single acoustic wave is used to diffract a pulsed input optical beam to produce the desired radiation pattern.